Semiconductor devices are processed by implanting ions into substrates. This processing requires tight control over the particles being directed toward the substrate. For example, in addition to the desired ions, neutral particles may be directed toward the substrate. These neutral particles may be deposited on the surface of the substrate, which may be detrimental to the ion implanting process for several reasons. First, their presence is not easily detected or controlled using traditional dosimetry techniques. In addition, these deposited neutral particles may alter the dopant/depth profile of the substrate. These neutral particles may also lead to outgassing of dopant during subsequent thermal processes.
These neutral particles reach the substrate by following the same or a similar path as the desired ions. FIG. 1 shows a representative ion implantation system, having an ion source 10, a process chamber 20, and a plurality of electrodes 30 which guide the ions along the desired path. The electrodes 30 may include suppression electrodes 31, disposed closer to the ion source 10 and ground electrodes 32, disposed away from the ion source. Additional sets of electrodes may also be utilized. In addition, each set of electrodes has a left electrode and a right electrode, with a space therebetween through which the ion beam passes. While these are referred to as left and right, they may also be upper and lower (for a horizontal ion beam). In each case, there are first and second opposed electrodes, where the ion beam passes therebetween. Thus, in FIG. 1, there are shown first and second opposed suppression electrodes 31, and first and second opposed ground electrodes 32. These electrodes 30 are in electrical communication with one or more power sources (not shown), which are used to apply an appropriate electrical voltage to each electrode 30. The positive ions are attracted through an aperture 11 toward the electrodes 30, which may be negatively charged. The aperture 11 is disposed in a wall of the ion source 10. The center of the aperture 11 serves as one end of a center line 36. The center line 36 is defined as a line perpendicular to the plane of the wall of the ion source 10 where the aperture 11 is disposed, and passing through the midpoint of the aperture 11. The ions may follow paths 35 as they travel to the substrate 40. In this embodiment, these paths 35 also are the light paths. Additionally, the electrodes 30 are disposed symmetrically about the center line 36. In addition, in this embodiment, the center line 36 also passes through the center of the substrate 40. Furthermore, in this embodiment, the central axis of the ion beam also corresponds to the center line 36.
As noted above, neutral particles may also exit the ion source 10, for example, due to pressure difference between the ion source 10 and the process chamber 20, and follow the path 36 toward the substrate 40. Also, ions in the ion source 10 can be accelerated by the early parts of the electric field, and then, while still in the high pressure of the plasma chamber, or in the throat of the aperture 11 lose their electric charge to another gas atom or molecule or by attracting an electron in the vicinity. Such a neutralized ion will be moving in generally the same direction as the ion beam, but are now neutral particles with much less energy because they have only been accelerated through a fraction of the potential drop from source to ground.
Since these neutral particles lack sufficient velocity, they are not implanted in the substrate, but rather are deposited on the surface of the substrate 40.
In addition, the plasma in the source chamber may typically a bright source of ultraviolet (UV) light. This UV light, which has sufficient quantum energy to cause chemical changes in oxides used in semiconductor devices, may be detrimental to the yield of the implantation process. The UV photons are themselves electrically neutral and are not deflected by electric fields.
Therefore, it would be beneficial if there were an apparatus and method to separate the flow of ions from the flow of neutral particles such that only ions reached the substrate. It would also be advantageous if this could be performed without the use of expensive equipment, such as mass analyzers.